1. Field of the Invention
This invention relates to methods of producing hydrocarbons from a well drilled through a subterranean formation. More particularly, a method for production of hydrocarbon and water through separate sets of perforations and injecting water into a disposal zone without producing the water to surface is provided.
2. Description of Related Art
Hydrocarbons, both liquid and gaseous, exist in the earth in many different types of rock strata. Some strata containing hydrocarbons are of very limited vertical extent and are bounded by impermeable layers. Production of the hydrocarbons from these thin strata often allows water (brine) to encroach naturally on the production wells, and when water production begins at a well the rate of water production increases rapidly and the wells are soon abandoned. In other thin strata, encroachment of water does not occur naturally and water is injected through selected wells into the strata to "flood" the hydrocarbons from the strata. Again, when water encroaches on a well, the well is usually abandoned soon thereafter. In these thin hydrocarbon reservoirs, there is little effect of gravity in separating the hydrocarbons and water in the reservoir, and the contact between hydrocarbon and water is at a substantial angle with respect to horizontal.
When a reservoir containing hydrocarbons is sufficiently thick, or production rate is sufficiently low, or permeability to flow in the rock is sufficiently high, or a combination of these conditions exists, the difference in density between hydrocarbons and water will cause a segregation of the fluids in the reservoir rock, with water being concentrated in the lower part of the reservoir. The oil-water contact zone will be approximately horizontal. If the dip angle of the reservoir is low, the oil-water interface will normally move up very slowly as hydrocarbons are produced from the reservoir and the contact zone between hydrocarbons and water may extend over the entire reservoir.
The zone between hydrocarbon and water is commonly referred to as the "oil-water contact" in a reservoir. The oil-water contact actually extends over a vertical distance in a reservoir, sometimes called a "transition zone," the distance of which is determined by fluid densities and capillary pressures in the rock. In a highly permeable rock, the transition zone is short, as little as a few inches, because capillary pressures are low. In extremely low permeability rock, the transition zone may extend over many feet. In the transition zone, the saturation of the pore spaces of the rock changes from predominantly water to predominantly hydrocarbon. At the highest saturations of hydrocarbons, water will not flow through the rock, being trapped in the rock by the hydrocarbons, and the permeability to flow of hydrocarbons is at its maximum value.
Although idealized sketches are prepared of fluid saturations in reservoirs and flow patterns in reservoirs, it is known in the art that properties of reservoir rocks vary widely within a hydrocarbon reservoir. Permeability and capillary pressures, in particular, vary from point-to-point, both in a vertical and a horizontal direction, but particularly in a vertical direction, as the strata often consist of beds having different properties that were formed under differing geologic conditions. These variations are difficult to impossible to predict with accuracy, and can only be measured by expensive techniques, such as analysis of cores cut from the rock around an individual well. Therefore, the saturation of hydrocarbons and water in the pore spaces of a reservoir is expected to vary in a somewhat unpredictable manner. Also, the relative amount of water produced with hydrocarbons into a well cannot be forecast with accuracy because of rock property variations when the well is producing from near the hydrocarbon-water interface in a stratum.
The location of the hydrocarbon-water contact near a production well may be affected by production from the well, because of the phenomenon of fluid "coning." Production above the oil-water contact may cause water to be drawn upward to the perforations and may lead to excessive water production from the well. Similarly, production of water below the oil-water contact tends to cause the contact to be pulled downward to the perforations.
When water and oil are produced through perforations in casing and up tubing in a wellbore, either by natural pressure or artificial lift methods, water often becomes emulsified in the oil. It is believed that a significant part of this emulsification occurs during simultaneous flow of the fluids through perforations. Therefore, minimizing mixing of water and oil in perforations will decrease the emulsification of fluids during production.
When water is produced from the earth along with hydrocarbons, it is normally pumped to the surface and disposed of by injection into another well. A disposal strata for the water must not contain water that is valuable, such as fresh water, and the disposal wells must not pose any threat to contamination of valuable water. The pumping of brine to the surface along with hydrocarbons and subsequent disposal through another well creates a risk of environmental pollution from such incidents as broken lines, spills, overflow of tanks and other occurrences. It would obviously be environmentally safer to dispose of the brine in the subsurface without producing it to the surface of the earth.
U.S. Pat. No. 4,766,957 discloses a method and apparatus for production of hydrocarbons with disposal of water produced along with hydrocarbons by producing the water and hydrocarbon from the same set of perforations in casing, separating water and hydrocarbon by gravity in or near the wellbore, and pumping the water to a lower water disposal zone as hydrocarbon flows to the surface.
U.S. Pat. No. 4,805,697 discloses a method and apparatus for production of hydrocarbons along with water from the same set of perforations by separating the water and hydrocarbon in the wellbore using a mechanical device and pumping the water to a disposal zone at a rate controlled by the content of oil in the separated water. U.S. Pat. No. 4,770,243 discloses specific apparatus for monitoring the content of oil in the separated water and controlling the rate of pumping of water into the disposal strata.
There is a long felt need for a method and apparatus for producing hydrocarbons from strata having a hydrocarbon-water interface lying substantially in a horizontal direction with a limited extent of transition zone by which the production of water and hydrocarbon can be controlled to minimize emulsification, while minimizing the effects of coning, and for disposing of the produced water in an environmentally safe manner.